Distributed fibre optic sensing is a technique for determining information about an optical fibre or about the environment around the optical fibre. The technique consists of sending one or more optical pulses along an optical fibre and detecting light reflected or backscattered from the fibre. The related technique of optical time domain reflectometry (OTDR) is commonly used in telecommunications for verifying the performance of optical fibre. For example, breaks in the optical fibre will be a site of reflection for the optical pulse. By measuring the time taken for the reflection of the input pulse to be received at the input end of the fibre, the distance along the fibre of the break can be determined.
Distributed fibre optic sensing may use coherent or partially coherent pulses, or essentially incoherent pulses. The technique may be used to determine detailed information about the fibre or optical system such as the occurrence of high loss regions, highly scattering regions, or to monitor changes in the optical properties of the fibre material. For example, changes in refractive index can be monitored. If the environment around the optical fibre imparts changes in the optical properties of the fibre, then changes in the environment can also be monitored.
WO 2005/116601, which is incorporated herein by reference, describes a method and apparatus for detecting pressure distribution in fluids. The apparatus includes a light source for transmitting pulses of light along a single mode optical fibre. Pulses are preferably linearly polarised and successively incident at two polarizations 45° to each other such that information on changes in the birefringence can be obtained. The polarisation of light backscattered from the fibre is detected. Pressure in fluids is isotropic and hence the pressure on an optical fibre in a fluid will act isotropically on the fibre. Such isotropic stress will try to compress the fibre uniformly in all directions. Hence, changes in the optical properties of the fibre will tend to be uniform and small. To use an optical fibre to measure the pressure in a fluid; the isotropic pressure must be converted to an anisotropic or asymmetric deformation of the fibre which can be detected by changes in birefringence. One prior art embodiment uses a type of fibre known as side-hole fibre (SHF) which has air holes spaced either side of the core and parallel to the fibre axis. Such side-hole fibre deforms asymmetrically under the influence of an isotropic pressure e.g. in fluid. For example, the core will experience a greater compressive strain perpendicular to the holes than parallel to the plane of the holes. The asymmetric deformation imparts an asymmetric stress on the core of the optical fibre changing its birefringence proportionally to the pressure. The change in birefringence is detected by the polarisation state of the backscattered light.
Although the side-hole fibre is effective for measuring isotropic pressure in fluids, the side-hole fibre is difficult and expensive to manufacture.
The side-hole fibre also has a maximum pressure range which can be measured by the fibre. The range is determined by the construction of the fibre.
Furthermore, as the measurable pressure range for a given fibre design increases, the absolute sensitivity of the fibre to changes in pressure decreases because a fractional change in birefringence is caused by an equivalent fractional change in pressure. In other words, for fibres with maximum pressure ranges of 1 MPa (10 bar) and 100 MPa (1000 bar) respectively, a 1% change in birefringence might be caused by a 1% change in pressure. This equates to 0.1 bar and 10 bar for the two fibres respectively resulting in the fibre having the 100 MPa pressure range having a much lower absolute sensitivity than the 1 MPa fibre. Hence, it would be desirable to provide an optical fibre that can measure large ranges of pressure to high sensitivity.